A typical transducer architecture of an electro dynamic speaker is shown in FIG. 1. The speaker comprises a diaphragm 1 connected to a voice coil former 2. The diaphragm is suspended by a first suspension 3, and the voice coil former is suspended by a second suspsension 4, also referred to as the spider. A voice coil 5 is arranged around the bottom of the former 2, in the gap of a magnet system. The entire system is embedded in a basket 6, provided with flanges 7, 8 for supporting the suspensions 3, 4.
When an alternating electrical current is brought to flow through the coil 5, a resulting force will move the diaphragm 1 and the former 2. The moving diaphragm moves the air and converts the electrical signal into an acoustical sound pressure waves.
The spider 4 and the suspension 3 form a two point suspension for the motion of the coil 5 and diaphragm 1. The closer these points are to each other, the less stable the system the system becomes to rocking modes. To build a rugged design that is insensitive for rocking modes a large distance of these points is desired. However, the distance between suspension and spider becomes lower with the total height.
Fort his reason, conventional speakers have the disadvantage that they are thick compared to their actual diaphragm displacement. Especially speakers for the low frequency range (woofers) need a strong suspension to provide a large diaphragm displacement without risking too much rocking of the system, and tus become thick. This is of course undesirable for applications like Flat Panel Televisions or placement in a door of a car.
On the other hand, a flat speaker design, with a small distance between the suspension points, will be less stable. This will have negative audible effects and it can even damage the system during operation.
Further, increasing the diameter of the coil has many electrical and thermal advantages to the system. However, increasing the diameter of the coil requires increasing the diameter of the coil former. In a conventional loudspeaker, where the spider is mounted to the peripheri of the coil former, this will result in a reduced effective spider area. A small spider limits the maximum excursion and requires a high force to stretch it. Furthermore it becomes non linear already for small displacements. The result is higher distortion and less sensitivity.
Document U.S. Pat. No. 6,819,773 discloses a speaker where the diaphragm extends through the coil former, and is suspended by the lower suspension (the spider). With this design, the voice coil is arranged between the suspension points, which allows for a slim design with improved stability. Also, the spider is connected to the diaphragm, and will not be affected by an increased coil diameter. However, a drawback with the system in U.S. Pat. No. 6,819,773 is that it requires a specific shape of the diaphragm, restricting the design freedom.